In case u r planning on laying a layer of soil on the roof, do opt for using pumice stone (lava with air bubbles trapped that cooled down super quick)... It's not easily found in nurseries. It's used in cloth factories for tumbling wash. There is a guy Patras who provides it. A pick-up loadful for 4K. 0331 4214208 . The stone weights less than a quarter of a similar size riverstone. It has great drainage, air trapped, and good water retention. It's recommended in ideal desert cactus mediums (60% pumice, 20 % top soil,20% sand)